The present invention is directed to a system and method for ultrasound imaging, and particularly imaging angular scatter by coherently processing data from multiple scattering angles using translating apertures.
Conventional ultrasound systems transmit pulses of high frequency sound into the body and map the magnitude of returned echoes to form B-mode images. The brightness of these images is a function of many factors including transmit and receive transducer geometry, attenuation and phase aberration in the propagation path, and most importantly, the acoustic scattering of the tissue itself. Conventional systems map the acoustic backscatter from tissue; that is the sound energy returned directly to the transmitter. While such images have great diagnostic value, they represent only a fraction of the information available from the scattered sound field.
One untapped source of information is angular scatter. As the incident wave scatters from tissue structures, different fractions of its energy are scattered in different directions. Angular scatter is described using the geometry shown in FIG. 1. In this nomenclature backscatter is indicated by a scattering angle of 180xc2x0, while angular scatter occurs at smaller angles. Although angular scatter information is not utilized clinically, it has been a topic of research for over a decade. Research in this area has consisted of both fundamental measurements and the development of practical imaging systems.
Angular scatter measurements have typically had the goal of measuring the average angular scatter over a large area, at a single frequency as discussed by W. J. Davros, J. A. Zagzebski, and E. L. Madsen, in Frequency-dependent angular scattering of ultrasound by tissue-mimicking materials and excised tissue, Journal of the Acoustical Society of America, vol. 80, pp. 229-237, 1986, and by J. A. Campbell and R. C. Waag, in Measurements of calf liver ultrasonic differential and total scattering cross sections, J. Acoust. Soc. Am., vol. 75, pp. 603-611, 1984, the entire disclosures of which are hereby incorporated by reference herein. These systems moved piston transducers mechanically around a target to interrogate different scattering angles. However, for reasons described below, these measurements exhibited large statistical fluctuations and therefore required significant averaging to yield reliable results. Thus, while these measurements lend insight into tissue scattering, their methods cannot be adapted for clinical imaging.
Previous angular scatter imaging systems have had the goal of imaging tissue at a single scattering angle other than 180xc2x0 as discussed by M. T. Robinson and O. T. V. Ramm, in Real-Time Angular Scatter Imaging System for Improved Tissue Contrast in Diagnostic Ultrasound Images, IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, vol. 41, 1994, and by J. C. Lacefield, in Angular Scatter Ultrasound Imaging using Separated Arrays, Duke University, 1999 the entire disclosures of which are hereby incorporated by reference herein. These systems have used one or more phased array transducers with the transmit aperture displaced physically from the receive aperture. By applying electronic focusing and beam steering, these systems were able to interrogate a 2-D region at high spatial resolution and with broad bandwidth. Angular scatter images were displayed beside accompanying B-mode images, however direct comparison was difficult because each image presented a different speckle pattern. While these systems have yielded interesting results, they do not coherently process data acquired at different scattering angles, and thus fail to make full use of angular scatter information.
FIGS. 15(A)-(D) illustrate k-space representations of a variety of angular scatter measurement/imaging geometries (k-space will be discussed in greater detail below). For introduction purposes, FIG. 15(A) indicates a simple backscatter geometry. The incident wave vector is indicated by xe2x80x9ci,xe2x80x9d the observed wave vector by xe2x80x9co,xe2x80x9d and the k-vector by xe2x80x9ck.xe2x80x9d The gray oval indicates the region of k-space interrogated by this system. This region is narrow in the axial spatial frequency dimension to indicate a narrow bandwidth. The lateral spatial frequency dimension is also narrow, indicating poor lateral spatial resolution.
FIG. 15(B) depicts the geometry used by Davros et al, as discussed above. The dark oval indicates the region of k-space interrogated by this system while the light oval is the backscatter system. Note the lack of overlap and thus lack of speckle coherence between the backscatter and angular scatter interrogation.
FIG. 15(C) indicates the k-space representation of the angular scatter system used by Campbell and Waag, as discussed above. The rotation of Davros is eliminated by rotating the transmitter and receiver by equal and opposite increments circumferentially. However, the downshift of the axial spatial frequencies has still eliminated any speckle coherence for this narrowband system.
There is therefore a need in the art for an effective system and method for ultrasound imaging. In particular, imaging angular scatter by coherently processing data from multiple scattering angles while still having stability in the speckle pattern with angle that allows direct comparison of echoes acquired at different scattering angles using a translating aperture.
Accordingly, FIG. 15(D) depicts the k-space representation of the translating apertures implemented on a broadband phased array system of the present invention. The broad bandwidth of this system ensures that some speckle coherence is maintained, even with the downshift in axial spatial frequencies.
According to the invention, a system is provided for imaging a target using imaging angular scattering comprising: a transducer array having a plurality of elements aligned along at least one of a plurality of translational axes wherein the plurality of translational axes are directed horizontally, vertically, and/or diagonally relative to the target; a transmitter for generating and transmitting ultrasound pulses at the target operably associated with the transducer array; a transmit aperture translator electrically associated with the transmitter for transmitting pulses to fire from the elements of the transducer array thereby defining a subject transmit aperture, wherein the subject transmit aperture comprises at least two preselected the elements; a receiver for receiving echoes of transmitted pulses operably associated with the transducer array and outputting echo signals therefrom; a receive aperture translator electrically associated with the receiver and for receiving pulses transmitted from the subject transmit aperture and received at the elements of the transducer array thereby defining a subject receive aperture, wherein the subject receive aperture comprises at least two preselected the elements; a controller for controlling the transmit aperture translator and the receive aperture translator wherein the transmission and reception are iteratively performed at least twice, wherein after each of the iterations of transmission and reception the subject transmit aperture and the subject receive aperture are translated along one of the plurality translation axes in a predetermined equal and opposite direction relative to one another; and a signal processor operably associated with the receiver, the signal processor adapted to receive the echo signals and perform angular scatter analysis on the echo signals after each of the second or subsequent iterations so as to provide an image signal representative of the target.
Further, the invention provides a translating apertures method of imaging a target comprising the steps of: a) providing a transducer array having a plurality of elements aligned along at least one of a plurality of translational axes wherein the plurality of translational axes are directed horizontally, vertically, and/or diagonally relative to the target; b) generating a subject ultrasound pulse at preselected elements of the plurality of elements to define a subject transmit aperture; c) focusing the subject ultrasound pulse to a predetermined point on the target; d) transmitting the subject ultrasound pulse from the subject transmit aperture to the target point; e) receiving echoes of the transmitted pulses at preselected elements of the plurality of elements to define a subject receive aperture; f) outputting echo signals received from the receive aperture; g) repeating step xe2x80x9caxe2x80x9d through step xe2x80x9cfxe2x80x9d at least one or more times, wherein after each repetition the method further comprises the additional step of: translating the subject transmit aperture and the subject receive aperture along one of the plurality of translation axes in a predetermined equal and opposite direction relative to one another; and h) processing the echo signals to perform angular scatter analysis on the echo signals after the first or subsequent repetitions so as to provide an image signal representative of the target.